1. Field of the Invention
The invention relates to an electrospray ionization (ESI) device, more particularly to a cycling electrospray ionization device that is adapted to produce liquid droplets of an electrospray medium in a moving manner.
2. Description of the Related Art
Through mass spectrometry, the molecular weights of analytes obtained from a sample can be obtained for identification of the analytes. A mass spectrometer generally includes an ionization device, a mass analyzer and a detector.
One ionization method is called electrospray ionization (ESI). As shown in FIG. 1, a conventional electrospray ionization device 11 performs an electrospray ionization procedure to ionize analytes contained in an electrospray solution. The conventional electrospray ionization device 11 includes a nozzle 112 having an open end 111 that opens toward an entrance side 121 of a mass analyzer 12 of an electrospray ionization mass spectrometer. When in use, an electric field, for instance, a 2˜5 kV potential difference, is established between the open end 111 of the nozzle 112 and the entrance side 121 of the mass analyzer 12. Subsequently, the electrospray solution is forced out of the nozzle 112 for traveling toward the open end 111. The electrospray solution forms a Taylor cone 2 that is filled with electric charges as it passes through the open end 111 of the nozzle 112 due to the combined effect of the electric field present between the open end 111 of the nozzle 112 and the entrance side 121 of the mass analyzer 12 and the surface tension of the electrospray solution at the open end 111. As the electric field force overcomes the surface tension of the electrospray solution at the open end 111 of the nozzle 112, liquid droplets containing multivalent electric charges and analytes are formed, and are forced to enter into the mass analyzer 12 through the entrance side 121 thereof.
As the charged droplets travel through the air from the open end 111 of the nozzle 112 toward the entrance side 121 of the mass analyzer 12, the liquid portion of the charged droplets vaporize such that the charged droplets dwindle in size, causing the multivalent electrons to attach to the analytes to form ionized analytes with relatively lower m/z values (i.e., the mass-to-charge ratio, where m is the mass of the ionized analyte, and z is the ionic charge/number of elementary charges). Since the molecular weight of a macromolecule, such as a protein molecule, is in the hundreds of thousands, charges attached to each of the macromolecules for forming the ionized molecules needs to be multivalent in order for the m/z value to be low enough so as to be detectable by the mass analyzer 12. Not only does the electrospray ionization method allow macromolecules to be efficiently ionized, but it also overcomes the detection limit imposed by the mass analyzer 12 since a lower m/z value can be obtained. Therefore, protein molecules can be studied using electrospray ionization mass spectrometry.
Several improvements have been developed for electrospray ionization in the past. As shown in FIG. 2, U.S. Pat. Nos. 6,350,617 and 6,621,075 disclose another conventional electrospray ionization device 11a including a rotary disk 113, and a plurality of nozzles 112 that are mounted on the rotary disk 13 and that are supplied respectively with a plurality of different electrospray sample solutions. The rotary disk 113 is rotatable, such that when it is required to perform electrospray ionization on a particular one of the electrospray sample solutions, a selected one of the nozzles 112 can be moved into a designated location relative to the mass analyzer 12 so as to permit the selected electrospray sample solution to enter into the mass analyzer 12. As shown in FIG. 3, U.S. Pat. No. 6,066,848 discloses another conventional electrospray ionization device 11b including an array of nozzles 112 respectively for spraying a plurality of different electrospray sample solutions, and a blocking device 114 adapted to be disposed between the nozzles 112 and the entrance side 121 of the mass analyzer 12 and formed with an aperture 115. The blocking device 114 is angularly movable relative to the nozzles 112 so as to permit the aperture 115 to be brought into alignment with a selected one of the nozzles 112. As a result, only the liquid droplets of the selected electrospray sample solution are permitted to pass through the aperture 115 thereby advancing toward the entrance side 121 of the mass analyzer 12 for mass analysis per each time. In other words, each of the conventional electrospray ionization devices 11a, 11b facilitates convenient electrospray ionization when multiple electrospray sample solutions are to be analyzed.
However, all conventional electrospray ionization devices, including those disclosed above, have the same disadvantage that during electrospray ionization of each single electrospray solution, the corresponding nozzle 112 is fixed in position when spraying the electrospray solution, such that only a portion of the ionized analytes will reach the mass analyzer 12 via the entrance side 121, while the other are dispersed into the surrounding environment due to the space charge phenomenon. As a result, intensity and stability of signals obtained by the mass analyzer 12 corresponding to the analytes are relatively low.
In view of the above, it would be significantly beneficial to the electrospray ionization mass spectrometry (ESI-MS) industry if the amount of ionized analytes reaching the mass analyzer 12 can be increased.